At 3 1/2 my daughter rode all rides you had to be 40 inches for. I think she was 4 when she rode allthe 42 inch rides. At 7 1/2 she has been on everything except mailiboomer. She is almost tall enough to ride Autopia by herself. The last time we went, they said if she wanted to ride alone they would let her even though she wasn't tall enough. I said no she will ride with her Aunt. Good thing too, because she tried the pedal (first time) and she was just a bit too short to keep the car going.
As for scary, you just never know, at 3 1/2 my daughter rode Tower of Terror. There was just no steering her clear of it. She didn't freak out, but hasn't ridden since. She doesn't really like Space Mountain, though she did enjoy it with the lights on, she doesn't do well not knowing which direction to go. We scream at the snowman on Matterhorn, she likes the ride, but not him, so will usually only go on it once in awhile. We hadn't been on Splash in a long time and she freaked out the last time we rode. Guess it's we won't be riding that for awhile. We were sitting in the back seat where two people can ride side by side. She used to do good until they made individual seats because she liked sitting in my lap in my arms. The last time her hands were wet on the bar and she was convinced going down the last drop she was going to fall out of the log. She was hysterical when she got off that she almost fell out of the log and that she couldn't hold on, and that she was never riding again. I felt the same way when I was a kid on the log ride at Knotts, so I understand her fear. I will try again in a few years and see how she feels.
I like thunder mountain, and think it's probably a good step up from gadgets go coaster. Nothing too scary jumping out at you (like the snowman on matterhorn). My daughter also didn't like Indy, too bumpy, and too much stuff jumping out at you.
Of course she also got concerned about Davy Jones in the Pirates ride, she thought he was a ghost and the boat was going through him. I had to tell her it was just a movie screen made out of fog, she turned around, realized what it was, and she's fine with it now.